The value of a sensor or transducer detecting a relative vibration between two mechanical members solely lies in its ability to selectively detect the relative vibration under detection without picking up other vibrations creating noise signals over-shadowing the signal representing the relative vibration under measurement. It is also crucially important that the sensor or transducer has a construction of solid structure that seals off the electrical components housed within the transducer body from the ambient surroundings exterior to the transducer body, whereby the sensor or transducer can be immersed into or exposed to hostile environments such as liquids and contaminating gases. For example, U.S. Pat. Nos. 4,888,991 and 5,054,322 issued on other inventions made by this inventor disclose embodiments of the effective and economic relative vibration sensor, which embodiments have an inherent limitation in applications as they cannot be immersed into or exposed to a hostile environment due to the lack of solid wall sealing off the electrical component from the hostile environments. The present invention teaches a piezo electric relative vibration sensor that selectively converts the relative vibration under detection into an alternating electrical signal, while rejecting the noise created by all other forms of absolute and relative mechanical vibration, which relative vibration sensor has a solid wall enclosing the electrical components and thus sealing off the electrical components from the ambient surroundings exterior to the relative vibration sensor.